The present invention relates to a process for post-treating roughened and anodically oxidized aluminum, in particular support materials for offset printing plates, with aqueous solutions.
Support materials for offset printing plates are provided, on one or both sides, with a radiation(photo-) sensitive layer (reproduction layer), either by the user directly or by the manufacturers of precoated printing plates. This layer permits the production of a printing image of an original by photomechanical means. After a printing form is thus produced from the printing plate, the image areas carried by the layer support accept ink in the subsequent printing process and, simultaneously, the areas which are free from an image (non-image areas) provide a hydrophilic image background for the lithographic printing operation.
For the above reasons, the following requirements are demanded of a layer support for reproduction layers used in the manufacture of offset printing plates:
Those portions of the radiation-sensitive layer which have become comparatively more soluble following exposure must be easily removable from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue. PA1 The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water rapidly and permanently during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink. PA1 The radiation-sensitive layer must exhibit an adequate degree of adhesion prior to exposure, and those portions of the layer which print must exhibit adequate adhesion following exposure. PA1 subjecting silicate layers on aluminum printing plate supports, which have been produced by an immersion treatment in aqueous alkali metal silicate solutions, to a hardening post-treatment with an aqueous solution of Ca(NO.sub.3).sub.2 or generally, with a solution of an alkaline-earth metal salt, in accordance with U.S. Pat. No. 2,882,153 and U.S. Pat. No. 2,882,154; as a rule, the alkaline-earth metal salt concentrations exceed 3% by weight. The support materials are roughened by chemical or mechanical means only, and no anodic oxidation takes place. PA1 German Offenlegungsschrift No. 22 23 850 (corresponding to U.S. Pat. No. 3,824,159) describes a process for coating aluminum moldings, aluminum sheets, aluminum castings or aluminum foils specifically for capacitors, but also for offset printing plates, in which an anodic oxidation is carried out in an aqueous electrolyte composed of an alkali metal silicate and an organic complexforming substance. The latter substance can be selected, for example, from amines, amino acids, sulfonic acids, phenols, glycols and, additionally, from salts of organic carboxylic acids, such as maleic acid, fumaric acid, citric acid and tartaric acid. PA1 The process for producing grain-like or textured surfaces on aluminum, according to German Auslegeschrift No. 26 51 346 (corresponding to British Pat. No. 1,523,030), is carried out directly on the aluminum, using alternating current, in an electrolyte which contains, in an aqueous solution, from 0.01 to 0.5 mol/l of a hydroxide or salt of an alkali metal or alkaline earth metal (e.g., a silicate) and, optionally, from 0.01 to 0.5 mol/l of a substance which forms a barrier layer. The German patent document discloses that the substances forming barrier layers include, among others, citric acid, tartaric acid, succinic acid, lactic acid, malic acid or the salts thereof. PA1 Aluminum support materials for offset printing plates in accordance with German Offenlegungsschrift No. 31 26 636 (corresponding to U.S. Pat. No. 4,427,765), which on an aluminum oxide layer produced by anodic oxidation carry a hydrophilic coating of a complex reaction product of (a) a water-soluble polymer, such as polyvinylphosphonic acid and (b) a salt of an at least bivalent metal cation, such as Zn.sup.2+, or PA1 the process in accordance with European Patent Application 0,089,510 (corresponding to U.S. Pat. No. 4,376,814), for producing aluminum support materials, in particular for offset printing plates, in which the usually anodically oxidized, sheet-like aluminum is post-treated, in a single-state process, with an aqueous solution containing (a) a sodium silicate, for example, and (b) a sodium salt or ammonium salt of a hydrophilic polymer, such as polyvinylphosphonic acid, which displays alkaline reactivity.
The preferred base material employed for layer supports of the above-described type is aluminum. More specifically, the aluminum is superficially roughened by means of known methods, such as dry brushing, wet brushing, sandblasting, chemical and/or electrochemical treatment. The roughened substrate then is optionally subjected to an anodizing treatment, during which a thin oxide layer is built up, in order to improve the abrasion resistance.
In practice, the support materials, particularly anodically oxidized support materials based on aluminum, are often subjected to a further treatment step before applying a radiation-sensitive layer, in order to improve the adhesion of the layer, to increase the hydrophilic properties and/or to improve the developability of the radiation-sensitive layer. Such treatments are, for example, carried out according to the following methods:
German Pat. No. 907,147 (corresponding to U.S. Pat. No. 2,714,066), German Auslegeschrift No. 14 71 707 (corresponding to U.S. Pat No. 3,181,461 and U.S. Pat. No. 3,280,734) and German Offenlegungsschrift No. 25 32 769 (corresponding to U.S. Pat No. 3,902,976) describe processes for hydrophilizing support materials for printing plates, which processes utilize aluminum which has optionally been anodically oxidized. In these processes, the materials are treated with an aqueous solution of sodium silicate, with or without the application of an electrical current.
From German Pat. No. 11 34 093 (corresponding to U.S. Pat. No. 3,276,868) and German Pat. No. 16 21 478 (corresponding to U.S. Pat. No. 4,153,461) it is known to use polyvinyl phosphonic acid or copolymers based on vinyl phosphonic acid, acrylic acid and vinyl acetate to hydrophilize support materials for printing plates based on aluminum which has optionally been anodically oxidized.
In accordance with European Patent Application No. 0,048,909 (corresponding to U.S. Pat. No. 4,399,021), it is possible to perform such a post-treating process not only by an immersion treatment, but also by means of electric current. According to the teaching of German Offenlegungsschrift No. 31 27 627 (corresponding to South African Pat. No. 82/4357), a polymer which can also be used in this context is polyvinylmethyl phosphonic acid.
Although these post-treating methods often yield satisfying results, they cannot meet all of the requirements, frequently very complex, demanded of a support material for printing plates to meet the standards now set for high-performance printing plates currently in use.
For example, a certain deterioration of the storability of reproduction layers applied must be accepted after the treatment of supports with alkali metal silicates which produce good developability and good hydrophilic properties. In supports which are treated with water-soluble organic polymers, the good solubility of these polymers, particularly in aqueous alkaline developers of the sort predominantly used for developing positive-working reproduction layers, leads to a decrease in the hydrophilic action. In addition, resistance to alkaline media, which is particularly required when high-performance developers are used in the field of positive-working reproduction layers, is not present to a sufficient degree. Depending on the chemical compositions of the reproduction layers, tinting in the non-image areas is occasionally encountered, which is probably caused by adsorptive effects.
In the prior art, modifications of the silicating processes and also treatment with hydrophilic polymers have already been described. Illustrative examples of these variations include:
The above-summarized modifications of hydrophilizing post-treatments with silicates or certain hydrophilic organic polymers, which can be employed for printing plate supports of aluminum, are incapable of producing surfaces of a quality suitable for high-performance printing plates. In particular, with respect to technological requirements, the layers either have not yet been improved to such an extent that they can fully satisfy the demands set forth above, or the processes for preparing different types of solutions having defined pH values and their control are too complicated and expensive.
German Offenlegungsschrift No. 32 32 485, which was not not pre-published, describes a process for post-treating roughened and anodically oxidized aluminum supports for printing plates, which is performed in two stages using (a) an aqueous alkali metal silicate solution and (b) an aqueous alkaline-earth metal salt solution.